Sciatic Nerve Compression and Piriformis Syndrome

41 Sciatic Nerve Compression and Piriformis Syndrome

Robert J. Spinner and Robert L. Tiel

Case Presentation

A 58-year-old woman with a long history of Crohn disease and rheumatoid arthritis, for which she was steroid dependent for years, presented with increasing left buttock pain for 2 years. She described a toothache-like pain that occasionally radiated distally and hypoesthesias in the left leg. Sitting, particularly on a firm, hard chair, or on the commode, aggravated her symptoms. She had no back pain.

Examination demonstrated pain in the left buttock region without tenderness in the left paraspinal lumbar area or lumbosacral area. There was mild sensory change in the L5–S1 distribution of the left foot but without weakness in the extremity. Ankle jerks were symmetric. Provocative maneuvers including internal rotation of the hip and straight and reverse straight leg raises caused striking buttock and leg pain.

Electrical studies showed mild neurogenic changes within the distribution of the sciatic nerve. This included chronic partial denervation in the tibialis anterior, tibialis posterior, and short head of the biceps. Bilateral H reflex studies were absent. The left peroneal and tibial motor responses were normal. The left tibial F-wave response was moderately prolonged and the left peroneal F-wave response was mildly prolonged.

Magnetic resonance imaging (MRI) of the lumbar spine showed moderate L4–5 disk desiccation only. MRI of the pelvis showed increased signal on T2-weighted images surrounding the left sciatic nerve that was of indeterminate significance, possibly due to inflammation. No abnormality was seen in the piriformis muscle.

Surgery was performed because of the chronicity of the symptoms and the ill-defined MRI findings. Through a posterior approach, the gluteus maximus fibers were mobilized as a flap. The sciatic nerve just distal to the notch had a flattened region, yet there was no identifiable compressive lesion. A group of abnormally dilated veins was seen just distal to the flattened area and to the piriformis. These vessels were coagulated and obliterated. The piriformis muscle was sectioned. A total neurolysis was performed. Normal nerve action potentials (NAPs) were recorded from both tibial and peroneal divisions.

Postoperatively, her condition remained unchanged. This case illustrates the fact that sciatic nerve surgery is not always gratifying. Sometimes when one has exhausted nonoperative therapy, a surgeon is forced into performing a largely “exploratory” operation.

Diagnosis

Mild sciatic neuropathy, questionably secondary to abnormal venous engorgement

Anatomy

The sciatic nerve is derived from the L4 through S3 nerve roots. The sciatic nerve runs in proximity to the piriformis muscle. The sciatic nerve typically runs between the piri-formis and the gemelli (Fig. 41–1). Variations of the sciatic nerve/piriformis complex are well known (Fig. 41–2). For example, the sciatic nerve or the tibial or peroneal component may split the piriformis. The piriformis may split the nerve into its two divisions. The sciatic nerve typically bifurcates into tibial and peroneal nerves in the distal thigh but may do so more proximally, including within the pelvis.

The sciatic nerve innervates most of the hamstrings and a portion of the adductor magnus near the ischial tuberosity or in the proximal thigh, but other branches to the short head of the biceps are given off more distally. The terminal branches supply all of the muscles below the knee. The sensory distribution of the sciatic nerve is completely below the knee and involves the entire foot except for the small medial zone supplied by the saphenous nerve.

The piriformis muscle divides the greater sciatic region into suprapiriformis and infrapiriformis regions. The superior gluteal nerve and vessels run in the suprapiriformis region. The pudendal neurovascular elements pass in the medial portion of the infrapiriformis region, and the inferior gluteal neurovascular bundle, the sciatic nerve, and the posterior cutaneous nerve of the thigh pass in the lateral part of the infrapiriformis region. The piriformis muscle connects the sacrum and the greater trochanter. Its main function is as an external rotator of the hip with the leg extended and abductor of the hip when the thigh is flexed.

Figure 41–1 The emergence of the sciatic nerve from the greater sciatic notch, under the piriformis muscle. (From Tiel RL. The surgical treatment of entrapment neuropathies of the lower extremity. Seminars in Neurosurgery 2001;12:109–123. Reprinted with permission.)

Characteristic Clinical Presentation

Sciatic nerve compression typically produces local buttock pain with radiating symptoms distally into the distribution of the sciatic nerve. Patients may complain of painful dysesthesias or paresthesias, or painless hypoesthesias. Their symptoms may be worse with prolonged sitting, especially on hard surfaces (i.e., so-called wallet neuritis or hip pocket neuropathy). Patients may also note subtle weakness in the foot or toes, which may affect their gait. In more extreme cases, they may experience severe motor dysfunction in all muscles below the knee and hamstrings. Patients typically do not have mechanical back or hip symptoms. Frequently they report a history of direct or indirect trauma to the buttock region.

Sciatic nerve irritation and compression can occur from the pelvis to the distal thigh. The most common site is between the greater sciatic notch and the ischial tuberosity. Sciatic nerve compression can occur due to scar (following surgery, trauma, infection, injection), mass (e.g., bony impingement, cement, tumor, abscess, hematoma, endometrioma), fibrous band, or vascular leash. Dynamic factors such as leg crossing may also play a role in tethering the nerve. Of course, direct injuries and stretch injuries to the sciatic nerve in the buttock region are well known and are discussed elsewhere (see Chapter 42).

Figure 41–2 The commonly observed relationships of the sciatic nerve to the piriformis muscle. In 78% it emerges under the piriformis muscle, in 21% the lateral division pierces the muscle before joining the medial division, and in 1% the lateral division splits above the piriformis muscle and joins the medial division inferiorly. (From Tiel RL. The surgical treatment of entrapment neuropathies of the lower extremity. Seminars in Neurosurgery 2001;12:109–123. Reprinted with permission.)

Far more controversial is the clinical entity of piriformis syndrome. Different theories exist to support the premise that the piriformis muscle itself either compresses or irritates the sciatic nerve. A widely held theory is that anatomical variations of the relationship of the sciatic nerve to the piriformis muscle predispose the sciatic nerve to entrapment by it. Some believe that when the sciatic nerve (or a division of it) passes through or posterior to the piriformis muscle, it may be vulnerable to entrapment lesions. Others believe these normal variations are coincidental findings rather than causally related. Some have postulated that piriformis spasm or degenerative or inflammatory changes in the piriformis cause compression against the bony pelvis and epineurial irritation. Part of the problem may be related to semantics. Purists claim that an irritated nerve in the vicinity of the piriformis muscle is not proof of a cause–effect relationship. In addition, they prefer to avoid the label syndrome and specify the cause, for example, sciatic nerve compression by synovial bursitis or posttraumatic scarring, rather than invoke a nondescript term (i.e., piriformis syndrome). As a result, some believe that the piriformis syndrome is underdiagnosed, many, overdiagnosed. Regardless of its prevalence, the diagnosis of piriformis syndrome should only be made after excluding all other potential causes of sciatica.

Another site of sciatic nerve entrapment has been described due to the fibrous edge of the biceps femoris at the level of the ischial tuberosity. This entrapment site has been diagnosed in athletes, especially with gluteal pain and sciatica.

Physical Examination

Sciatic nerve lesions may produce demonstrable sensory disturbance or motor weakness in the sciatic nerve distribution. In chronic cases, trophic changes may be present on the plantar aspect of the foot. Ankle and hamstring reflexes may be reduced in the affected limb, whereas the quadriceps reflex is preserved. The presence of percussion tenderness may help localize the lesion. A mass lesion may be palpated. Examination of the lumbosacral spine, sacroiliac joint, hip, and vasculature should be normal.

Patients with sciatic nerve compression near the sciatic notch typically have localized pain between the posterior superior iliac spine and the greater trochanter. Frequently the diagnosis of piriformis syndrome is made without firm objective findings, and the diagnosis is largely a clinical one. Sometimes, piriformis spasm may be demonstrable on rectal exam. There may be lateral pelvic wall pain on rectal or pelvic examination, and this may produce sciatic nerve paresthesias. Provocative tests that stretch the piriformis include forced hip internal rotation with the thigh extended; Freiberg sign (Fig. 41–3), resistance to abduction and external rotation; Pace sign (Fig. 41–4), voluntary adduction, flexion, internal rotation of the hip; positive Lasègue sign; and knee extension with forward bending. Hip external rotation may lessen pain.